Telegraphic keying circuit



Feb. 22, 1938. J.. D, WALLACE 2,108,833 lTELEGRAPl-IIC KEYING CIRCUIT Filed July 22, g1950 /y ed v /90 l INVENTOR. Jaume/.s @.Qf/aecuw,

@ m5; .NM

ATTORNEY Patented Feb. 22, 1938 UNITED STATES PATENT Fries 2 Claims.

(Grantee under the act of March 3, 188s, as

amended April 30,

My invention relates broadly to signaling systems and more particularly to a telegraphic keying circuit for signaling systems.

One of the objects of my invention is to provide a simplified circuit arrangement for a telegraphic keying circuit i'na signaling system having means for suppressing the key clicks normally offering objectionable interference in nearby receivers by the reproduction of noisesin the nature of thumps.

` Another object of my invention is to provide an electron tube circuit yarrangement for a modulation system in a telegraphic transmitter having means for preventing abrupt changesof vpotential of the grid with respect to the cathode for eliminating key clicks in the transmitted signals.

Still another object of my invention is to provide an arrangementof screen grid tube circuit including a filter circuit adapted to suppresskey clicks during rapid telegraphic modulation' of a signal transmitter.

Other and further objects of my invention will be understood from the speciiication hereinafter following by reference to the accompanying drawing in which:

Figure l shows a telegraphic modulation cir-- cuit embodying the principles of my invention; Fig. 2 illustrates a modified form of keying circuit having means for suppressing key clicks; Fig. 3 shows a further arrangement of keying circuit which may be employed according to my invention; Fig. 4 shows a noise suppressing circuit connected in the keying system according to a further modied form of my invention; and Fig. 5 illustrates another modified arrangement of circuit for eliminating key clicks according to my invention. i

Referring in detail to Fig. 1 of the drawing, the keying circuit of my invention includes a screen grid electron tube indicated by reference character l having cathode 2, control'grid 3, plate electrode 4, and screen grid 5. The input circuit is indicated at 5 containing a source of sustained oscillations l. The tuned output circuit is shown at 8. 'Ihe cathode 2 is heated from suitable source 2a. The plate potential is fixed by battery il and the screen grid potential is fixed by a tap i3 on the battery Il. By-pass condensers I9 and 29 are arranged around the battery l'i and the tapped sections thereof. The battery 9 is provided tor supplying bias potential.' Reference character iii indicates a potentiometer composed of two resistances lua and lb. Reference character Il indicates a keying resistor, and l2 shows the key.

The keying action is obtained as follows: When l2 is open, the voltage of the control grid 3 with respect to the cathode 2 is equal to the battery voltage 9. This should be great enough to block the tube. When i2 is closed, resistance li is paralleled with part oi the potentiometer lil. Under this con-dition, the control grid is con,- nected across part of the potentiometer it instead oi the entire battery, and the negative control grid potential with respect to the cathode is decreased. In this manner the transmitter may be telegraphically keyed according to the signaling code. The condenser it, and resistance l5 cooperate in suppressing the key clicks. Some rectied grid current will flow through the resistor it?, but this has no effect on its action of removing key clicks. Before the grid potential with respect to the filament may be changed, a certain quantity of electricity must flow' either into or from the condenser i4, depending on whether the control grid potential with respect to the cathode is raised or lowered. Since no electrons flow from the control grid to filament during the proper operation of an amplifier, it is obvious that the charging or discharging condenser current must new through resistance l5. it may be shown from the laws of the electric circuit that under the above conditions the voltage across the condenser it will not change instantaneously, but will require a certain period of' time for its potential to change. Since this condens-er l is directly across the cathode and control grid of the aniplier, and assuming that the generator 'l has a low impedance to waves produced by keying disturbances, then the potential of the grid with respect to the cathode will not change as abruptly as it would, if the condenser li and resistance l5 were omitted. Very objectionable key clicks are produced by the circuit of Fig. 1 ii the condenser i@ and resistance l5 are omitted.

Another method of keying sometimes used in connection with screen tubes consists of alternately applying and removing the positive potential from the screen grid. A condenser and resistance may be connected in the circuit of the screen grid to aid in the elimination of key clicks. The circuit shown in Fig. 2 is an example of utilizing capacity and resistance in the screen grid circuit to lessen the effect of key clicks. The keying action of this circuit is as follows: The key li when open does not allow any positive potential with respect to the cathode to reach the screen grid 5, and experience has shown that when the screen grid potential is removed from a screen grid tube, no electronic current ows from cathode to plate, When I2 is closed, positive potential reaches the screen grid through the resistors II and 2|, thereby causing plate current to flow. 'I'he action`of th-e resistances II and 2| and condenser I4 toward the elimination of key clicks is as follows: When I2 is closed, the screen grid potential does not instantaneously rise to its proper value, but the condenser 22 must receive a charge before it assumes the proper potential, and since this charging current must flow through resistance 2|, a finite time is required to charge Ythe condenser and consequently, the screen grid cannot assume its proper potential until the condenser I4 is charged. When I2 is opened, before the screen grid loses its positive charge, the condenser I4 must discharge. Since I4 must discharge through the resistance II and the resistance of the electronic path from, cathode to screen grid, the discharge is not instantaneous, but'a finite time is taken for the discharge. Therefore, it has been shown that the use of the condenser Id and the resistances II and 2 I cause the change in screen grid voltagev not to be abrupt, but cause the change to be gradual. The

Vplate battery II need not necessarily be tapped as shown at I8, but a potentiometer may be utilized, or a series resistance of proper value may be used which would enable the entire plate battery I'I to be connected across the circuit supplying the screen grid 5. Key clicks in a five electrode tube may be suppressed in this manner, keying the screen grid, or space charge grid, or both. The circuit Will function in connection with certain tubes having a high shield grid to cathode resistance with the resistance H, short circuited and removed from circuit. The condenser 22 is a radio frequency by-pass from the screen grid 5 to the cathode 2, and has so low a value of capacity, that its effects are inconsequential on the operation of the key click eliminator.

The circuit in Fig. 3 shows another method of screen grid keying with the circuit for the elimination of key clicks. The keying action of the circuit in Fig. 3 is as follows: When the key I2 is open, the proper potential reaches the screen grid through the three resistances I I, 2| and 23, but when I2 is closed the current flows through 23 directly to the cathode I, thereby leaving the screen grid at cathode potential,and the electronic current from cathode to plate then ceases. The total resistance of the threeV resistors VI I, 2|, and 23 must be of great enough value to cause enough drop from the screen grid current, so that the screen grid voltage Will be of the proper value. Also 23 must be of great enough value to prevent too much drain on the plate supply source Il when the key I2 is closed. The action of condenser I4 and resistancesV II and 2| as an eliminator1 of key clicks will be apparent from the circuit arrangement. When key I2 is opened, time is required to charge the condenser I0 through resistances 2| and 23, and therefore the screen grid 5 does not instantaneously get its operating potential. When the key I2 is closed, the condenser I4 then discharges through two paths, I and cathode to screen, and 2|, and because of the resistance it doesnot discharge immediately, and therefore the screen grid does not lose its positive potential immediately. It has therefore been shown that the use of the resistances Iii and 2| and the condenser I4 eliminates key clicks by the prevention of very abrupt changes in screen grid potential. The condenser 22 in Fig. 3 has the same function that it had in Fig. 2. The potential supply for the screen grid may be supplied from a tapped battery, or from a potentiometer, instead of from the entire plate Vbattery I'I as shown. Also a ve electrode tube may be used in connection with this keyA click eliminator, keying either screen grid, space charge grid, or both. The circuit shown in Fig. 3 may also have resistance II removed and still function properly with certain types of tubes.

There is another method of keying a screen grid tube. Instead of merely removing its positive potential, its potential is changed to negative with respect to the filament to key the output of the transmitter. A method. of doing this is shown in Fig. 4. When the key-l2 is open, the screen grid obtains its potential from the plate battery through the'k resistance 2|. When I2 is closed the screen grid is connected to the bias battery, and when the screen grid is at a negative potential the electronic flow from filament toV plate ceases. But when the screen grid has changed from positive to negative, the condenser I4, since it is connected from screen grid to cathode, has also changedits polarity in like manl ner. But the charging or discharging current must flow through resistance II. As a result the condenser charge does not change instantly, but takes a nite time. In this way the screen grid does not change its potential so abruptly, and therefore key clicks are eliminated. When key I2 has been closed, and is. then opened, the condenser |4 and resistance II act again so as to prevent'the change in screen grid potential from being so abrupt. The capacity 22 keeps the screen grid at the radio frequency potential of the cathode, and has so low a value of capacity that it has no effect on the action of the key click eliminator. 1

A circuit which operates similarly to the circuit of Fig. 4 is shown in Fig. 5. The resistance I I is located in a slightly different position in the circuit as shown. However, in using this method, II must have much less resistance than 72|, or these resistors may become a potentiometer which may cause the screen grid to have a negative potential not nearly so large as the voltage of battery 9 when the key I2 is closed, and it is possible in certain cases, that the screen grid would be slightly positive underV this condition. It is obvious that a ve electrode tube could be used inY place of a four electrode tube, keying either shield grid or space charge grid, or both.

The values of resistance used in connection with the key click eliminator vary from about 5000 to 20,000 ohms. The capacity used varies from about 0.1 microfarad to y0.5 microfarad. These values given Vapply to keying systems which are hand operated. Their values must be reduced if high speedY automatic keying is usedQ The invention herein described and claimed may be used and/or manufactured by or for the Government of the United States of America for governmentalV purposes without the payment of any royalties thereon or therefor.

,Vi/hile I have described. my invention in Ycertain preferred embodiments. I desire that it be understood that various modications of the circuit arrangement of my invention Vmay be employed and I intend no limitations upon my invention other than are imposed by the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is as follows:

1. In a telegraphic keying system', an` electron tube having a cathode, an anode, a control grid and a screen grid, an input circuit interconnecting said cathode and control grid, an output circuit interconnectingl said cathode and anode, a keying circuit comprising a source of potential and three resistors connected in seriesv between said screen grid and said cathode, a key connected to short-circuit one of said resistors and said source, and a condenser connected between said cathode and the terminal remote from said screen grid of the resistor nearest said screen grid.

2. In a telegraphic keying system, an electron tube having a cathode, an anode, a control grid and a screen grid, an input circuit interconnecting said control grid and said cathode and comprising a source of biasing potential connected to apply a negative biasing potential to said control grid, an output circuit interconnecting said cathode and anode, a keying circuit comprising a key, a condenser and a resistance connected in series between said cathode and the negative terminal of said source of biasing potential, said screen grid being connected to a point in said keying circuit on the side of said condenser remote from said cathode, and a source of potential and a resistor connected in series between said cathode and. said screen grid for applying to said screen grid a potential positive with respect to said cathode but of magnitude smaller than said source of biasing potential, whereby during keying said screen grid alternately has positive and negative potential with respect to said cathode.

JAMES D. WALLACE. 

